Apparatus and method of arrhythmia detection in a subcutaneous implantable cardioverter/defibrillator

ABSTRACT

In a subcutaneous implantable cardioverter/defibrillator, cardiac arrhythmias are detected to determine necessary therapeutic action. Cardiac signal information is sensed from far field electrodes implanted in a patient. The sensed cardiac signal information is then amplified and filtered. Parameters such as rate, QRS pulse width, cardiac QRS slew rate, amplitude and stability measures of these parameters from the filtered cardiac signal information are measured, processed and integrated to determine if the cardioverter/defibrillator needs to initiate therapeutic action.

CROSS-REFERENCE TO CO-PENDING APPLICATIONS

This application is a continuation of application Ser. No. 09/990,510,filed Nov. 21, 2001, now U.S. Pat. No. 6,754,528.

FIELD OF THE INVENTION

The subject invention relates generally to implantablecardioverter/defibrillators and, more particularly, to detection ofcardiac arrhythmias with subcutaneous implantablecardioverter/defibrillators.

BACKGROUND OF THE INVENTION

Defibrillation/cardioversion is a technique employed to counterarrhythmic heart conditions including some tachycardias or fast heartrhythms originating in the atria and/or ventricles. Typically,electrodes are employed to stimulate the heart with electrical impulsesor shocks of a magnitude substantially greater than pulses used incardiac pacing. A variety of shock waveforms are used for bothdefibrillation and pacing, including truncated exponentially decayingmonophasic and biphasic waveform pulses as well as pulses where thewaveform maintains a relatively constant current over the duration ofdelivery to the myocardium.

Defibrillation/cardioversion systems include body implantable electrodesthat are connected to a hermetically sealed container housing theelectronics, battery supply and capacitors. The entire system isreferred to as an implantable cardioverter/defibrillator (ICD). Theelectrodes used in ICDs can be in the form of patches applied directlyto epicardium, or, more commonly, the electrodes are located on thedistal regions of small cylindrical insulated catheters that typicallyenter the subclavian venous system, pass through the superior vena cava,and into one or more endocardial areas of the heart. Such electrodesystems are called intravascular or transvenous electrodes. U.S. Pat.Nos. 4,603,705; 4,693,253; 4,944,300; and 5,105,810, the disclosures ofwhich are all incorporated herein by reference, disclose intravascularor transvenous electrodes, employed either alone, in combination withother intravascular or transvenous electrodes, or in combination with anepicardial patch or subcutaneous electrodes. Compliant epicardialdefibrillator electrodes are disclosed in U.S. Pat. Nos. 4,567,900 and5,618,287, the disclosures of which are incorporated herein byreference. A sensing epicardial electrode configuration is disclosed inU.S. Pat. No. 5,476,503, the disclosure of which is incorporated hereinby reference.

In addition to epicardial and transvenous electrodes, subcutaneouselectrode systems have also been developed. For example, U.S. Pat. Nos.5,342,407 and 5,603,732, the disclosures of which are incorporatedherein by reference, teach the use of a pulse monitor/generatorsurgically implanted into the abdomen and subcutaneous electrodesimplanted in the thorax. This system is far more complicated to use thancurrent ICD systems using transvenous lead systems together with anactive canister electrode, and therefore, it has no practical use. Ithas, in fact, never been used because of the surgical difficulty ofapplying such a device (3 incisions), the impractical abdominal locationof the generator and the electrically poor sensing and defibrillationaspects of such a system.

Recent efforts to improve the efficiency of ICDs have led manufacturersto produce ICDs that are small enough to be implanted in theinfraclavicular pectoral region, a site allowing access to thesubclavian venous system. In addition, advances in circuit design haveenabled the housing of the ICD to form a subcutaneous electrode. Someexamples of ICDs in which the housing of the ICD serves as an optionaladditional electrode are described in U.S. Pat. Nos. 5,133,353;5,261,400; 5,620,477; and 5,658,321 the disclosures of which areincorporated herein by reference.

ICDs are now an established therapy for the management oflife-threatening cardiac rhythm disorders, primarily ventricularfibrillation (VF) and also ventricular tachycardia (VT). ICDs are veryeffective at treating VF and VT, but traditional ICD implantation stillrequires significant surgery and surgical skill, especially regardinglead insertion into the venous system and lead positioning in the heart.

As ICD therapy becomes more prophylactic in nature and used inprogressively less ill individuals, including children, the requirementof ICD therapy to use intravenous catheters and transvenous leads is amajor impediment to very long-term management as most individuals willdevelop complications related to lead system malfunction, fracture orinfection sometime in the 5- to 10-year time frame, often earlier. Inaddition, chronic transvenous lead systems, their removal andreimplantation, can damage major cardiovascular venous systems and thetricuspid valve, as well as result in life-threatening perforations ofthe great vessels and heart. Consequently, use of transvenous leadsystems, despite their many known advantages, are not without theirchronic patient management limitations in those with life expectanciesof >5 years. The problem of lead complications is even greater inchildren where body growth can substantially alter transvenous leadfunction and lead to additional cardiovascular problems and revisions.Moreover, transvenous ICD systems also increase cost and requirespecialized interventional rooms and equipment as well as special skillfor insertion. These systems are typically implanted by cardiacelectrophysiologists who have had a great deal of extra training.

In addition to the background related to ICD therapy, the presentinvention requires a brief understanding of a related therapy, theautomatic external defibrillator (AED). AEDs employ the use of cutaneouspatch electrodes, rather than implantable lead systems, to effectdefibrillation under the direction of a bystander user who treats thepatient suffering from VF with a portable device containing thenecessary electronics and power supply that allows defibrillation. AEDscan be nearly as effective as an ICD for defibrillation if applied tothe victim of ventricular fibrillation promptly, i.e., within 2 to 3minutes of the onset of the ventricular fibrillation. AEDs, unlike ICDs,only make shock/no-shock decisions, as they are relieved of the burdenof needing to deliver complicated tiered therapeutic responses that theICD encumbers as a consequence of detecting and treating a multitude ofrhythm problems with a multitude of therapies. AEDs either shock for alife-threatening event or they do not shock. ICDs, on the other hand,are designed for a variety of interventions and use a differenttechnological approach for arrhythmia detection, redetection, assessmentof effectiveness and therapy.

AED therapy has great appeal as a tool for diminishing the risk of deathin public venues such as in airplanes. However, an AED must be used byanother individual, not the person suffering from the potential fatalrhythm. It is more of a public health tool than a patient-specific toollike an ICD. Because >75% of cardiac arrests occur in the home, and overhalf occur in the bedroom, patients at risk of cardiac arrest are oftenalone or asleep and cannot be helped in time with an AED. Moreover, itssuccess depends to a reasonable degree on an acceptable level of skilland calm by the bystander user.

What is needed, therefore, for life-threatening arrhythmias, especiallyfor children and for prophylactic long-term use for adults at risk ofcardiac arrest, is a novel combination of the two forms of therapy whichwould provide prompt and near-certain defibrillation, like an ICD, butwithout the long-term adverse sequelae of a transvenous lead systemwhile simultaneously harboring most of the simpler diagnostic andtherapeutic technological approaches of an AED. What is also needed is acardioverter/defibrillator that is of simple design and can becomfortably implanted in a patient for many years.

Further, an ICD is needed that can detect various types of cardiacarrhythmias to provide a patient with adequate therapy according to thetype of cardiac arrhythmia experienced by the patient. Although ICDshave multiple secondary functions, they primarily have two keyfunctions: detection and therapy of life-threatening cardiacarrhythmias. ICDs constantly monitor a patient's cardiac activity andanalyze the cardiac activity to determine the appropriate therapy thatshould be delivered to the patient. The type of therapies available tobe delivered to the patient include pacing, which can be single or dualchamber, therapy to correct slow heart rates or bradycardia calledanti-bradycardia pacing, therapy to correct slow and moderately fastventricular tachycardia (VT) and sometimes atrial tachyarrhythmias,called anti-tachycardia pacing (ATP), and therapy to correct ventricularfibrillation (VF) or high energy shocks. Thus, given the various typesof therapies available, it is very important to classify the type ofcardiac arrhythmias appropriately.

Detection schemes of cardiac arrhythmias are characterized using twoindexes of performance: sensitivity and specificity. Sensitivitygenerally refers to the ability of the detection scheme or algorithm toaccurately detect an abnormal heart rhythm for which the physiciandesires the device to treat. Specificity generally refers to the abilityof the detection scheme or algorithm to not treat rhythms that thephysician determines the device should not treat, such as sinustachycardia. Sensitivity values for VT/VF are typically between 90% to98%. There is constantly a tradeoff between the ability to detect anabnormal rhythm and the desire to prevent treatment of a normal rhythm.The higher the sensitivity, the more likely the detection algorithm willresult in an inappropriate therapy of a normal rhythm and, thus, lowerspecificity. The higher the specificity, the less likely the device willbe able to detect rhythms that should be treated and, thus, the lowerthe sensitivity. Despite a desire to limit false positive interventions,specificity values are typically only between 70% to 90% in order to notmiss therapy of a life-threatening disorder. In practice, there is aconstant tension between sensitivity and specificity.

Factors influencing detection algorithm performance include hardwareperformance, lead electrode placement, electrode design, electrodeshape, inter-electrode spacing and the analytical capabilities anddesign of the detection scheme itself. Because ICDs are battery poweredand, therefore, an unlimited supply of power is not available to performcomplex and power intensive analytical functions, the energy required toperform the analytical functions is also a factor influencing detectionscheme performance.

Another factor that influences the sensitivity and specificityperformance of a detection algorithm is the clinical balance betweenfalse negatives and false positives for each arrhythmia that thedetection algorithm is required to detect. The more arrhythmias requiredto detect, the more overlap and complexity in the algorithm. Forexample, one might require a very high sensitivity for fast VT/VF, givenits lethality, and therefore accept a lower specificity compared to therelatively benign arrhythmia of atrial fibrillation (AF).

A false negative therapeutic decision results from a detection schemethat calls a bona fide treatable event “normal”. But, depending upon thearrhythmia, some false negatives are more tolerable than others. On theother end of the spectrum is a false positive therapeutic decision thatresults from a detection algorithm that calls a “normal” event an“abnormal” event, thus inappropriately indicating that therapy should bedelivered. False positives for therapy of VT are more clinicallyacceptable than false positives for rhythms like AF where the former isimmediately life-threatening and the later is seldom so.

A false negative can result in a missed arrhythmia and can lead todeath, and a false positive may result in an inappropriate shock thatwill be uncomfortable, but not life threatening. Therefore, it isunderstandable that detection algorithms are skewed to result in falsepositives and reduce the occurrence of false negatives to zero forrhythms like fast VT but are more common for rhythms like AF where lowersensitivity is more clinically acceptable. Performance of typicaldetection schemes in current devices result in approximately 15% to 45%false positives for life-threatening disorders like VF and fast VT.

Typically, ICDs primarily use a rate-based classification scheme. Thatis, the intervals between successive heart beats are measured and,depending on their values, they are classified as slow, normal, or fast.Slow heart beats are treated with pacing (i.e., anti-bradycardiapacing), when the rate reaches a critically low level, according to aphysician's direction via the programming of the device. Programmableparameters for slow heart rates include primarily rate and hysteresis.Normal heart rates are left alone, where normal is usually defined inthe range of 40-100 bpm. Fast heart beats are often further classifiedinto three zones with various therapies ascribed to each zone. Forexample, the lowest zone may have a series of anti-tachycardia pacing(ATP) therapies maneuvers programmed for rhythms like monomorphic VTthat may fall in this lowest tachycardia rate zone. The next higher zonemay integrate a limited number of ATP attempts with a moderate energyshock therapy of approximately between 5-10 Joules for that zone.Finally, the highest zone may have a scheme programmed to deliver thehighest output shock energy for rhythms falling in this zone having VFor VT with the fastest and therefore most life-threatening rates.

Additional rate-based qualifiers have been used to improve thespecificity of detection schemes. Examples of such qualifiers includeparameters like sudden onset to eliminate false positive detection ofsinus tachycardia and heart rate interval stability to eliminate falsepositive detection of rapid AF for VT. Sudden onset refers totachyarrhythmias that are a result of a precipitous increase in heartrate as opposed to sinus tachycardia where the higher rates tend to comeon gradually. The term stability in detection algorithms usually, butnot always, refers to the coupling interval between heart beats as a wayto distinguish the characteristically irregular interval-to-intervaltime sequence of AF from more regular cardiac rhythms like sinustachycardia or monomorphic ventricular tachycardia where theinterval-to-interval time sequence is more regular and rarely varies bymore than 30 ms unlike AF where variation by 40 ms or more is the rule.Although typically faster in rate, VF does, like AF, exhibitinterval-to-interval instability. It can be distinguished from AF in adetection algorithm mostly by its typically much faster rate but also byoverlaying other measures in addition to rate and interval-to-intervalstability that result from an examination of the electrocardiographicQRS features.

The term stability, when used in detection algorithms, can have anothermeaning and refer to electrocardiogram (ECG) QRS signal stability, i.e.,the ability of the QRS to have identical or at least very similar signalcharacteristics on a beat-to-beat basis. In one example of stability,one can examine QRS duration or width, QRS amplitude, QRS slew rate(rate of change of the voltage signal), QRS signal template matching,and/or QRS signal frequency content as an indicator of beat-to-beatstability. VF, for example, under this example of stability would behighly unstable as the QRS is constantly changing in all of the waysindicated above in this paragraph. Confirming the presence of VF,regardless of rate, would aid dramatically in applying a shock/no-shockdecision-making process.

Recent developments in QRS morphology measurements involve a scheme inwhich certain areas within a cardiac QRS complex are measured andcompared against a normal set as defined by the physician at the time ofimplant or as defined automatically by the device during presetfollow-up measurement intervals.

There are other issues that enter into the ability of ICDs to detectabnormal heart rhythms depending on the cardiac information that isactually presented to the device's sensing electrodes (which mayrepresent only part of the complete set of cardiac information at anypoint in time), the ICD energy available to analyze the information, thetime required or allowed to make the analysis, and the ability or lackthereof for the device to modify itself over time with respect tochanges in the patient's underlying cardiac disease process or even suchshorter duration events as changes in body positioning.

With respect to limitations in cardiac information available to thedevice's sensing electrodes, current devices use a first electrode atthe tip deep inside the right ventricle as one half of an electrodesensing pair. The second electrode of the pair can be a shocking coilalso in the right ventricle and in close proximity to the firstelectrode, a second cylindrical electrode approximately 0.5 to 2 cm awayfrom the first tip electrode, or the metallic housing of the deviceitself. Because the first electrode is in close proximity to cardiactissue, it is more sensitive to electric fields present in the tissueclosest to it and less sensitive to the entire electric field generatedby all of the cardiac tissue. This contrast to a subcutaneous ICDsensing pair that has the advantage of a true far-field cardiac signal.

With respect to the energy limitations of current devices, the abilityto process and analyze cardiac signals depends on the energy availableto the devices. Current devices are limited on the amount of energyavailable for analyzing signals because the devices must reserve asignificant portion of the energy for pacing and shocking over thecourse of several years. For example, one measure of a device'sperformance is its ability to filter out unwanted signals, this iscalled common mode rejection ratio (CMRR). The higher the CMRR of adevice, the better able it is to reject signals common to bothelectrodes. Current devices have a CMRR of about 50 db. Energy isrequired to achieve higher CMRR values. Another aspect of the analysisrequiring energy is the use of a microprocessor with appropriatesoftware inside the device to analyze cardiac information. Continuousrunning of a microprocessor in an implantable device would result in arapid battery drain with unacceptably short battery longevity. As aresult, current devices reserve a portion of hardware dedicated to crudeanalysis of the underlying rhythm on a continuous basis and onlyactivate the microprocessor when further analysis is needed. Onceactivated, the microprocessor, along with its software programming,further analyze the cardiac information, decide on a course of actionand, when finished, go back to inactive mode.

For a subcutaneous only ICD, these issues of arrhythmia detectionsensitivity and specificity, false positives and false negatives,battery depletion, and algorithm design require significantly differentconsiderations and unique hardware considerations and algorithm designcompared to typical ICDs with 1-2 sensing leads in immediate contactwith the myocardium.

What is then needed is an implantable cardioverter/defibrillator thathas different sensing capabilities to ensure that an ICD using onlysubcutaneous shocking and sensing electrodes performs at acceptablelevels as well as has the ability to adapt itself to changes over time.

SUMMARY

According to one aspect of the invention, an apparatus and method areprovided for detecting cardiac arrhythmias with a subcutaneousimplantable cardioverter/defibrillator. The method includes the steps ofsensing cardiac signal information from far field electrodes implantedin a patient, amplifying the sensed cardiac signal information,filtering the amplified cardiac signal information, measuring parameterssuch as rate, amplitude, QRS pulse width, cardiac slew rate, QRS signalfrequency content, QRS morphology, and stability measures of theseparameters, and processing and integrating the parameters of the cardiacsignal information to determine if the cardioverter/defibrillator needsto initiate therapeutic action.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference is now made tothe drawings where like numerals represent similar objects throughoutthe figures where:

FIG. 1 is a block diagram of a cardiac arrhythmia detection schemeaccording to an embodiment of the present invention;

FIG. 2 is a diagram showing various heart zones for a cardiac arrhythmiadetection scheme according to an embodiment of the present invention;

FIG. 3 is a flow diagram illustrating the interrelation of the detectionscheme of FIG. 1 in an S-ICD according to an embodiment of the presentinvention; and

FIG. 4 is a diagram showing a shock/no-shock boundary for a cardiacarrhythmia detection scheme according to a two-dimensional embodiment ofthe present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A flow diagram of an algorithm for detecting various types of cardiacarrhythmias is shown in FIG. 1 and is discussed below. The detectionalgorithm of the present invention has the ability to detect and treatventricular rhythm disorders and withhold treatment for supraventriculararrhythmias such as AF. In certain embodiments, the detection algorithmof the present invention can be employed by a Subcutaneous ImplantableCardioverter-Defibrillator (S-ICD) or a Unitary Subcutaneous ImplantableCardioverter-Defibrillator (US-ICD), such as those described in U.S.patent applications entitled “SUBCUTANEOUS ONLY IMPLANTABLECARDIOVERTER-DEFIBRILLATOR AND OPTICAL PACER,” having Ser. No.09/663,607, now U.S. Pat. No. 6,721,597; and UNITARY SUBCUTANEOUS ONLYIMPLANTABLE CARDIOVERTER-DEFIBRILLATOR AND OPTIONAL PACER,” having Ser.No. 09/663,606, now U.S. Pat. No. 6,647,292, of which both applicationswere filed Sep. 18, 2000, and the disclosures of both applications arehereby incorporated by reference. Although the algorithm is intended forthe detection and therapy of life-threatening rhythm disorders like fastVT and VF, it may also be adapted for use in the treatment of AF (orother rhythm disorders) by accepting, rather than rejecting, for therapythe diagnosis of AF.

One feature of S-ICD and US-ICD devices that facilitates cardiac rhythmmonitoring is the size and location of the lead. Far field cardiacsignals are more easily detected by use of larger electrodes. The S-ICDand US-ICD devices typically use electrode size much larger than thatused in standard ICD electrode systems for sensing VT/VF and thus aremore appropriate for far field ECG sensing. In addition, the location inthe 3^(rd) to 12^(th) rib space (such as between the 4^(th) to 6^(th)rib space in one example), provides a sufficient signal for detection ofthe cardiac rhythm with use of a subcutaneous-only ICD location.Examples of such S-ICD devices and electrodes that provide propershapes, sizes and locations for the devices and electrodes are describedin U.S. patent application “SUBCUTANEOUS IMPLANTABLECARDIOVERTER-DEFIBRILLATOR EMPLOYING A TELESCOPING LEAD,” U.S. Ser. No.10/011,941, now U.S. Pat. No. 7,043,299 and “SUBCUTANEOUS ELECTRODE WITHIMPROVED CONTACT SHAPE FOR TRANSTHORACIC CONDUCTION,” U.S. Ser. No.10/013,980, now U.S. Pat. No. 7,065,410, of which both applications werefiled Nov. 5, 2001 and the disclosures of both are hereby incorporatedby reference.

In addition, S-ICD and US-ICD devices typically provide an electrodethat is inward facing toward the heart to facilitate improved arrhythmiadetection and rhythm monitoring. This design of the electrodecompensates for not being directly in contact with the heart, like thesensing electrodes of the common transvenous ICD. The compensatingfeatures of this type of lead electrode for the S-ICD allow betterrhythm detection by facing the myocardium and simultaneously avoiding orminimizing exposure to surface artifacts and noise that may be easilydetected by subcutaneous sensing electrodes.

In addition, the S-ICD and the US-ICD devices have two or moreelectrodes that provide a far-field view of cardiac electrical activitythat includes the ability to record the P-wave of the electrocardiogramas well as the QRS. In one embodiment of the present invention, thedetection algorithm can detect the onset of AF by referencing to theP-wave recorded during normal sinus rhythm and monitoring for its changein rate, morphology, amplitude and frequency content as well as itstiming and polarity relationship to the QRS. For example, a well-definedP-wave that abruptly disappears and is replaced by a low-amplitude,variable morphology signal would be a strong indication of the absenceof sinus rhythm and the onset of AF. In an alternative embodiment of thedetection algorithm, the ventricular detection rate could be monitoredfor stability of the R-R coupling interval. In the examination of theR-R interval sequence, AF can be recognized by providing irregularlyspaced R-R intervals due to variable conduction of atrial beats to theventricles through the atrioventricular node. An R-R interval plotduring AF appears “cloudlike” in appearance when many R-R intervals areplotted over time and compared to sinus rhythm or other supraventriculararrhythmias. Moreover, a distinguishing feature of AF when compared toventricular fibrillation is that the QRS morphology is similar on abeat-by-beat basis during AF despite the irregularity in the R-Rcoupling interval. In yet another embodiment, AF may be detected byseeking to compare the timing and amplitude relationship of the detectedP-wave of the electrocardiogram to the detected QRS (R-wave) of theelectrocardiogram. Normal sinus rhythm has a fixed relationship that canbe placed into a template-matching algorithm that can be used as areference point should the relationship change.

In other aspects of the AF detection process, one may includealternative electrodes that may be brought to bear in the S-ICD orUS-ICD systems either by placing them in the detection algorithmcircuitry through a programming maneuver or by manually adding suchadditional electrode systems to the S-ICD or US-ICD at the time ofimplant or at the time of follow-up evaluation. One may also useelectrodes for the detection of AF that may or may not also be used forthe detection of ventricular arrhythmias, given the different anatomiclocations of the atria and ventricles with respect to the S-ICD orUS-ICD housing and surgical implant sites.

Once AF is detected, therapy can be withheld since AF is relativelybenign or the arrhythmia can be treated by delivery of a synchronizedshock using energy levels up to the maximum output of the device therapyfor terminating AF or for terminating other supraventriculararrhythmias. Synchronization can be conducted by detecting the onset ofthe QRS signal and delivering the atrial cardioversion shock during thesynchronized time period. The S-ICD or US-ICD electrode system can beused to treat both atrial and ventricular arrhythmias not only withshock therapy but also with pacing therapy. In a further embodiment ofthe treatment of AF or other atrial arrhythmias, one may be able to usedifferent electrode systems than what is used to treat ventriculararrhythmias. Another embodiment would be to allow for different types oftherapies (amplitude, waveform, capacitance, etc.) for atrialarrhythmias compared to ventricular arrhythmias.

An algorithm for detecting various types of cardiac arrhythmias with anS-ICD or a US-ICD will be described below with respect to FIG. 1. Itshould be noted that although the following description refers to anS-ICD that employs the detection algorithm, the description also appliesto a US-ICD. As described above, the S-ICD may be positioned between thethird rib and the twelfth rib of a patient and uses a lead system thatdoes not directly contact the patient's heart or reside in theintrathoracic blood vessels of the patient.

It is also known that while S-ICD and US-ICD devices are able to pace,the devices are not intended for long-term pacing. Typically, the S-ICDprovides short-term pacing in the order of a few hours as an emergencytherapy so that the patient can consult with a physician or othercare-giver. As a result, some of the energy that would be used forpacing is available for better processing and sophisticated analysis ofthe sensed cardiac signals. Accordingly, the S-ICD has energy availableto achieve CMRR values of approximately 70 db to 100 db, in one example.

As noted above, S-ICDs use signals gathered from far field electrodes asopposed to electrodes that are in contact with the heart of a patient.As a result, a more sophisticated set of features can be used to make adevice with higher specificity performance characteristics. A variety ofdetection schemes are possible with both the information available bythe electrode placement as well as with the processing power available.

In addition to the use of the sense circuitry for detection of atrialand ventricular information, sense circuitry can check for the presenceor the absence of respiration. The respiration rate and/or minute volumecan be detected by monitoring the impedance across the thorax usingsubthreshold currents delivered across the active can and the highvoltage subcutaneous lead electrode (or the 2 can electrodes in oneUS-ICD embodiment) and monitoring the frequency in undulation in thewaveform that results from the undulations of transthoracic impedanceduring the respiratory cycle. If there is no undulation, then the patentis not respiring and this lack of respiration can be used to confirm theQRS findings of cardiac arrest. The impedance measurement using theS-ICD and US-ICD electrodes can also be used to extract stroke volume,ejection fraction or cardiac output measures providing a hemodynamicsensor input that can be combined with rate, QRS signal frequencycontent, QRS morphology, and stability measures of these parameters tofurther refine the shock/no-shock decision-making process.

The activity level of the patient can also be detected using miniature1-D, 2-D or 3-D accelerometer. Piezo-electric crystals can also be usedas a sensor to determine the activity level of the patient. Activitylevel of the patient can be another input parameter used by thedetection algorithm to further refine the shock/no-shock decision-makingprocess.

Even the patient's position can be detected by using a miniature mercuryswitch that is actuated when the patient is horizontal. The patient'sposition can be used as another input parameter to the detectionalgorithm to further refine the shock/no-shock decision-making process.

Referring now to FIG. 1, a block diagram of a cardiac arrhythmiadetection scheme according to an embodiment of the present invention isillustrated. The scheme uses elements comprising an electrocardiograminput 12, an amplifier 14, a narrow-band filter 16, a wide-band filter18, a QRS detector 20, an analog-to-digital (A/D) converter 22, an R-Rinterval detector 24, a QRS width detector 26, a peak amplitude detector28, a slew rate detector 30, an R-R interval stability detector 32, aQRS width stability detector 34, a peak amplitude stability detector 36,a slew-rate stability detector 38, additional post processor 40, and aparameter integrator 42.

Electrocardiogram (ECG) 12 comprises cardiac signal information sensedby the far field electrodes placed subcutaneously. In an embodiment, thesignal information is sent to amplifier 14, which has a CMRR value of atleast approximately 80 db. The signal information is then sent fromamplifier 14 to narrow-band filter 16 and wide-band filter 18. In anexample, narrow-band filter 16 has corners set at approximately 10 Hzand 30 Hz, and wide-band filter 18 has corners set at approximately 1 Hzand 50 Hz.

In an embodiment, from the narrow-band filter 16, the filtered signalinformation is presented to QRS detector 20 for purposes of identifyingthe timing of the QRS complex. From the wide-band filter 18, thefiltered signal information is presented to A/D converter 22 forconverting the signal information to a digital format.

From QRS detector 20, the signal information is presented to R-Rinterval detector 24, QRS width detector 26, peak amplitude detector 28and/or slew rate detector 30. R-R interval detector 24 measures the timeinterval between each successive QRS complex. The signal informationfrom R-R interval rate detector 24 can be further presented to R-Rinterval stability detector 32, which analyzes the variability betweeneach successive QRS complex. QRS width detector 26 measures the timeinterval of the sampled QRS complex. The signal information from QRSwidth detector 26 can be further presented to QRS width stabilitydetector 34, which analyzed the variability between the width of eachsuccessive QRS complex. Peak amplitude detector 28 measures the maximumamplitude of the sampled QRS complex. The signal information from peakamplitude detector 28 can be further presented to peak amplitudestability detector 36, which analyzes the variability between the peakamplitudes of each successive QRS complex. Slew rate detector 30measures the slew rate of the sampled QRS complex. The signalinformation from slew rate detector 30 can be further presented to slewrate stability detector 38, which analyzes the variability between theslew rates of each successive QRS complex.

In an embodiment, after processing by R-R interval detector 24, R-Rinterval stability detector 32, QRS width detector 26, QRS widthstability detector 34, peak amplitude detector 28, peak amplitudestability detector 36, slew rate detector 30 and/or the slew ratestability detector 38, the signal information can be presented toadditional post processing 40 for further processing such as firstderivative processing, second derivative processing, etc.

In an embodiment, from A/D converter 22, the digitized cardiac signalinformation is further analyzed with respect to parameters including QRScomplex width by QRS width detector 26, QRS peak amplitude by peakamplitude detector 28, and/or rising edge slew rate of the QRS complexby slew rate detector 30. From the QRS width detector 26, the peakamplitude detector 28, and the slew rate detector 30, the signalinformation can be further processed by additional post processors 40.Examples of additional processing may include adaptive filtering toavoid the slow-term effects that drugs or other substrate changes mayhave on the parameters, X out of Y filtering to avoid decisions based onsingle events, moving average filters, etc. In addition, firstderivative processing, second derivative processing, etc., may also beaccomplished.

In one embodiment of the detection algorithm, rate is measured on acontinuous basis. Other parameters may have registers associated forfine tuning by a programmer and/or determined periodically orcontinually on a beat-by-beat basis during normal sinus rhythm (NSR) andused to enhance the detection algorithms effectiveness by compensatingfor the relatively slow-changing effects many of the input parametersmay have due to body positional changes, substrate changes and drugeffects.

For example, short-term or long-term NSR averages of QRS width andR-wave amplitude would help compute the variability of these parameterswith greater insensitivity to cardiac substrate changes.

It is also envisaged that many if not all of these parameters will bestored on a beat-by-beat basis in a circular buffer, allowing moresophisticated algorithms to scan back in time and take into account thepast behavior of the system.

In addition, to conserve power consumption, a predetermined thresholdwill be used to enable the advanced analysis when multiple inputvariables are evaluated. Several parameters such as rate, cardiac QRSpulse width, cardiac QRS slew rate, and the various measures of signalstability are, in one embodiment, measured on a continuous basis. Inanother embodiment, the non-rate measures may be turned on only when therate exceeds a predetermined limit. The range of this predeterminedlimit could be 50-240 bpm. In one embodiment, the limit for activationof the non-rate detection measures would be 140 bpm. In anotherembodiment, the predetermined threshold that allows advanced analysismay be a measure other than rate, for example, QRS width or some othermeasure. In any case, each of these parameters has associated registersfor fine tuning or modifying the parameters on either a periodic basisor a more dynamic basis such as that associated with body positionalchanges.

FIG. 2 is a diagram showing various heart rate zones for a cardiacarrhythmia detection scheme according to an embodiment of the presentinvention. FIG. 3 is a flow diagram illustrating the interrelation ofthe detection scheme of FIG. 1 and the heart rate zones of FIG. 2according to an embodiment of the present invention.

In one embodiment of the detection algorithm, rate is used as a firstcriterion in the detection algorithm. As shown in FIG. 2 and FIG. 3,four rate zones are defined, two of which can overlap each other. Thefirst zone, defined as rates faster than Rate 1, define the fast VT andVF zone. Rates greater than Rate 1 will result in charging of thecapacitors and immediate shock therapy. Rates between Rate 1 and Rate 2are defined as a sustained VT/VF zone. Rates in this zone will only betreated if they persist for a pre-determined period, (for example, 20seconds), even if the output of the parameter integration block 42 mightindicate treatment should be withheld. The sustained rate zone can bedisabled by effectively setting Rate 1 and Rate 2 equal. Rates betweenRate 1 and Rate 3 define an SVT discrimination zone. Rates within theSVT discrimination zone enable further processing by the parameterintegration block 42 shown in FIG. 1. The desired output of theparameter integration block 42 is a decision to apply shock treatment orwithhold shock treatment due to non-shockable rhythms such as SVTs likeAF. Finally, rates below Rate 3 are defined as the NSR zone. In anembodiment, these rates are not treated.

The parameter integration block 42 combines n-parameters to distinguishshockable rhythms from non-shockable rhythms. Numerous methods are wellknown in the art to process such n-parameters and to determine a desiredsensitivity and specificity based on patient testing data. Some of thesemethods include statistical classifiers, fuzzy logic, artificial neuralnetworks and rule-based systems. In operation, each of these methodsconstructs an integration block which can distinguish shockable andnon-shockable rhythms from n parameters, or n dimensions. In oneexample, a statistical classifier can describe the decision boundaryusing only rate and QRS width stability criteria. These metrics orfeatures are used to calculate scalar numbers or vectors that summarizethe characteristics of the signals.

FIG. 4 is a diagram showing a shock/no-shock boundary for a cardiacarrhythmia detection scheme according to a two-dimensional embodiment ofthe present invention. More specifically, FIG. 4 shows two differentclasses of data from a graph relating QRS width stability to rate. Oneclass of data identifies an SVT condition that should not be shocked.The other class identifies a VT/VF condition that should be shocked. Asshown in FIG. 4, the two classes of data are separated by a decisionboundary line. The decision boundary line can be drawn empirically, ordetermined analytically by computing the distance each feature vectorlies with respect to the mean, weighted by the standard deviations ofthe class mean. These distances are known as the Mahanalobis distances.The decision boundary line can then be determined as the lineequidistant from the means or centroids of the two classes in an attemptto separate the two classes of rhythms with the least misclassificationerror. In three dimensions, different classes can represent multipleclusters. Again, either a plane or surface plot can be determinedempirically or analytically by computing Mahanalobis distances of thedifferent classes using set of patient testing data. The decision toshock or not-shock is then determined by observing on which side of theboundary the feature vector lays.

In another embodiment, the decision to shock/no-shock can be performedwith as few as one parameter. For example, a single scalar value can beset using QRS width. Beats with a QRS width wider than a threshold areclassified as ventricular tachycardia. The threshold maximum may befixed or may be adjusted based on NRS computed measures or othercalculations to account for long-term cardiac substrate changes.

In another embodiment, a single parameter, such as rate, can bemonitored such that once it reaches a threshold, the detection algorithmcan then analyze other parameters to determine the shock/no-shockdecision, thus conserving battery power.

In another embodiment, the decision to shock/no-shock can be performedwith three parameters. For example, in three dimensions, one axisrepresents QRS stability, another axis represents QRS slew-rate and thethird axis represents rate. The decision boundary can then berepresented as a surface, which can be a simple plane or a more complexthree-dimensional surface. The decision boundary surface can be computedanalytically or empirically to determine the shock/no-shock decision.Such a surface could also be updated and changed according to acontinuous or semi-continuous or periodic measurement of referencevalues for that patient.

While a two-dimensional embodiment of the detection algorithm is shown,other embodiments are possible such as an n-dimensional embodiment thatemploys n different input parameters.

Artificial neural networks can also be used to determine theshock/no-shock decision boundary by emulating the observed properties ofbiological nervous systems and drawing on the analogies of adaptivebiological learning. The principal feature of artificial neural networksis a structure of highly interconnected processing elements that areanalogous to neurons and are tied together with weighted connectionsthat are analogous to synapses. Learning involves the changes that occurto the synaptic connections between neurons.

Neural networks are well suited to process n-dimensional inputparameters and can be trained to produce a shock/no-shock decision oftenwith imprecise or noisy data. Many different neural structures arepossible. For example, one structure comprises a multilayer perceptronthat is trained with a backpropagation of error algorithm, althoughnumerous other structures and training algorithms could also beeffective.

Numerous characteristics and advantages of the invention covered by thisdocument have been set forth in the foregoing description. It will beunderstood, however, that this disclosure is, in many aspects, onlyillustrative. Changes may be made in details, particularly in matters ofshape, size and arrangement of parts without exceeding the scope of theinvention. The invention's scope is defined, of course, in the languagein which the appended claims are expressed.

1. A method of monitoring cardiac function of a patient in animplantable cardiac treatment system, the method comprising: obtaining acardiac rate for the patient from subcutaneous electrodes spaced apartand subcutaneously implanted in the patient; determining whether thecardiac rate: (a) exceeds a first threshold but does not exceed a secondthreshold; or: (b) exceeds the second threshold; and: if the cardiacrate exceeds the second threshold, directing therapy to the heart; or ifthe cardiac rate exceeds the first threshold but does not exceed thesecond threshold, directing further analysis to determine whethertherapy is indicated.
 2. The method of claim 1, wherein therapy isdelivered using the same electrodes used for capturing a signal forobtaining the cardiac rate.
 3. The method of claim 1, wherein the secondthreshold includes both a rate threshold and a time threshold, whereinthe second threshold is exceeded if the cardiac rate exceeds the ratethreshold for a period of time in excess of the time threshold.
 4. Themethod of claim 1, wherein, if the cardiac rate does not exceed thefirst threshold, therapy is withheld.
 5. The method of claim 1, wherein:the step of performing further analysis includes receiving a signalcorresponding to a portion of a cardiac complex and analyzing the signalusing a microprocessor; and if the cardiac rate does not exceed thefirst threshold, the signal is not analyzed by the microprocessor. 6.The method of claim 1, wherein the first threshold is set to about 140bpm.
 7. The method of claim 1, wherein the step of directing therapy tothe heart includes charging a capacitor system and delivering a shockbetween first and second subcutaneously implanted electrodes disposedexclusive of the patient's heart.
 8. The method of claim 1, wherein theimplantable cardiac stimulus system includes a canister housingoperational circuitry configured to perform method steps and a leadassembly coupled to the canister and electrically connecting theoperational circuitry to at least one stimulus electrode, wherein thestep of directing therapy to the heart is performed such that a stimulusis delivered using a single electrode on the lead assembly and a singleelectrode on the canister.
 9. The method of claim 8, wherein the leadassembly does not contact the patient's heart.
 10. A method of operatingan implantable cardiac treatment system, the method comprising:obtaining a cardiac rate for a patient using two implanted electrodesdisposed outside of the heart and vasculature of the patient;determining whether the cardiac rate: (a) exceeds a first threshold butdoes not exceed a second threshold; or: (b) exceeds the secondthreshold; and: if the cardiac rate exceeds the second threshold,directing therapy to the heart; or if the cardiac rate exceeds the firstthreshold but does not exceed the second threshold, directing furtheranalysis to determine whether therapy is indicated.
 11. The method ofclaim 10, wherein the step of directing therapy to the heart includes:charging a capacitive network to a desired voltage; and discharging thecapacitive network between shocking electrodes disposed outside of theheart and vasculature of the patient, at least one of the shockingelectrodes being disposed on a canister containing operational circuitryfor operating the system.
 12. The method of claim 11, wherein the secondthreshold includes both a rate threshold and a time threshold, whereinthe second threshold is exceeded if the cardiac rate exceeds the ratethreshold for a period of time in excess of the time threshold.
 13. Themethod of claim 11, wherein: the step of performing further analysisincludes receiving a signal corresponding to a portion of a cardiaccomplex and analyzing the signal using a microprocessor; and if thecardiac rate does not exceed the first threshold, the signal is notanalyzed by the microprocessor.
 14. The method of claim 11, wherein thefirst threshold is set to about 140 bpm.
 15. The method of claim 11,wherein: the second threshold includes both a rate threshold and a timethreshold such that the second threshold is exceeded if the cardiac rateexceeds the rate threshold for a period of time in excess of the timethreshold; if the cardiac rate does not exceed the first threshold,therapy is withheld; the step of performing further analysis includesreceiving a signal corresponding to a portion of a cardiac complex andanalyzing the signal using a microprocessor; and if the cardiac ratedoes not exceed the first threshold, the signal is not analyzed by themicroprocessor.